Also fixes:
MDEV-11331 Wrong result for INSERT INTO t1 (datetime_field) VALUES (hybrid_function_of_TIME_data_type)
MDEV-11333 Expect "Impossible where condition" for WHERE timestamp_field>=DATE_ADD(TIMESTAMP'9999-01-01 00:00:00',INTERVAL 1000 YEAR)

This patch does the following:
1. Splits the function Item::save_in_field() into pieces:
- Item::save_str_in_field()
- Item::save_real_in_field()
- Item::save_decimal_in_field()
- Item::save_int_in_field()

2. Adds the missing "no_conversion" parameters to
   Item::save_time_in_field() and Item::save_date_in_field(),
   so this parameter is now correctly passed to
   set_field_to_null_with_conversions().
   This fixes the problem reported in 11333.

3. Introduces a new virtual method Type_handler::Item_save_in_field()
   and uses the methods Item::save_xxx_in_field() from the implementations
   of Type_handler_xxx::Item_save_in_field().

These changes additionally fix the problem reported in MDEV-11331,
as the old code erroneously handled expressions like
COALESE(datetime-expression) through the STRING_RESULT branch of
Item::save_in_field() and therefore they looked like string type expressions
for the target fields. Now such expressions are correctly handled by
Item::save_date_in_field().

The full list of functions moved:
int case_stmt_action_expr(LEX *, Item* expr);
int case_stmt_action_when(LEX *, Item *when, bool simple);
int case_stmt_action_then(LEX *);
bool add_select_to_union_list(LEX *,bool is_union_distinct,  bool is_top_level);

This is a preparatory change for "MDEV-10142 PL/SQL parser",
to reuse the code easier between sql_yacc.yy and coming soon sql_yacc_ora.yy.

MDEV-11294 Move definitions of Derivation, DTCollation, Type_std_attributes from field.h and item.h to sql_type.h

Some upcoming tasks, e.g.:
- MDEV-10577 sql_mode=ORACLE: %TYPE in variable declarations
- MDEV-10914 ROW data type for stored routine variables

will need to reuse the code implemented in prepare_create_field(),
sp_prepare_create_field(), prepare_blob_field().

Before reusing this code, it's a good idea to move these global functions
as methods to Column_definition.

This patch:
- actually moves prepare_create_field(), sp_prepare_create_field(),
  prepare_blob_field() as methods to Column_definition
- makes sp_prepare_create_field() call prepare_create_field() at the end,
  to avoid duplicate code in MDEV-10577 and MDEV-10914.
- changes the return data type for prepare_create_field() from int to bool,
  to make it consistent with all other functions returning "ok" or "error".
- moves the implementation sp_head::fill_field_definition() from sp_head.cc
  to sp_head.h, as it now uses globally visible Column_definition methods,
  and is very simple, so inlining is now possible.
- removes the unused "LEX*" argument from sp_head::fill_field_definition()

There was a duplicate code to create TYPELIB from List<String>:
- In typelib() and mysql_prepare_create_table(), which was used to initialize
  table fields.
- create_typelib() and sp_prepare_create_field(), which was used to initialize
  SP variables.
create_typelib() was incomplete and didn't check for wrong SET values.

Fix:
- Moving the code from create_typelib() and mysql_prepare_create_field()
  to news methods Column_definition::create_interval_from_interval_list()
  and Column_definition::prepare_interval_field().
- Moving the code from calculate_interval_lengths() in sql_table.cc
  to a new method Column_definition::calculate_interval_lengths(), as it's now
  needed only in Column_definition::create_interval_from_interval_list()
- Reusing the new method Column_definition::prepare_interval_field() in both
  mysql_prepare_create_table() and sp_prepare_create_field(), instead of the
  old duplicate code pieces
- Removing global functions typelib() and create_typelib()

This patch also fixes:
MDEV-11155 Bad error message when creating a SET column with comma and non-ASCII characters
The problem was that ErrCongString() was called with a wrong "charset" parameter.

        JSON depth calculation fixed.

        String insertion fixed.

        The Item_func_json_contains_path was mistakenly set with the
        no '*' paths limitation.

MDEV-11562 Assertion `js->state == JST_VALUE' failed in check_contains(json_engine_t*, json_engine_t*).

        check_contains() fixed. When an item of an array is a complex
        structure, it can be half-read after the end of the recursive
        check_contains() call. So we just manually get to it's ending.

in slow shutdown mode don't stop purge threads until they've
purged everything there is

in slow shutdown mode stop all bg threads that might generate
new undo records to purge before stopping purge threads.

in slow shutdown mode purge threads really must exit only when there is
nothing to purge. Restore the trx_commit_disallowed check and
don't stop purge threads until all connection thread transactions
are gone.

The patch for bug mdev-10882 tried to fix it by providing an
implementation of the virtual method build_clone for the class
Item_cache. It's turned out that it is not easy provide a valid
implementation for Item_cache::build_clone(). At the same time
if the condition that can be pushed into a materialized view
contains a cached item this item can be substituted for a basic
constant of the same value. In such a way we can avoid building
proper clones for Item_cache objects when constructing pushdown
conditions.

* rpm upgrade fix
* update test results
* valgrind warning

        Now JSON functions return warnings if arguments are invalid.

name innodb background threads as such

MDEV-11518 Assertion `!table || (!table->read_set || bitmap_is_set(table->read_set, field_index))' failed in Field_long::val_int()

QUICK_RANGE_SELECT::init_ror_merged_scan() should
also set and restore TABLE::vcol_set

Aria supports virtual columns, but does not support indexes
on virtual columns. Let's issue an appropriate error in this case.

strangely enough, ?: variant does not link in some older gcc versions:

sql/sql_table.cc:6409: undefined reference to `Alter_inplace_info::ALTER_STORED_GCOL_EXPR'
sql/sql_table.cc:6409: undefined reference to `Alter_inplace_info::ALTER_VIRTUAL_GCOL_EXPR'

by introducing new Item::precedence() method and using it
to decide whether parentheses are required

* do setup/cleanup for charset and arena only once per table
  not for every vcol
* don't copy every vcol expression into table's memroot for parsing,
  do it in a temporary String buffer that is freed at the end
* add asserts

now, when expr_str is gone, expr_item can be unambiguously
renamed to expr.